The present invention relates to diamond substrates having piezoelectric thin films, and to methods of their manufacture, utilized for surface-acoustic wave devices and optical-device materials employable in high-frequency bands.
Surface-acoustic wave devices exploiting surface-acoustic waves, (abbreviated SAWs hereinafter), which propagate over solid-body surfaces, are miniature, lightweight and highly temperature-stable, and have various features such as being superior in phase characteristics. Accompanying multi-channel/high-frequency transformations in mobile communications and satellite communications fields in recent years have been calls for the development of surface-acoustic wave devices employable in high-frequency bands; methods that employ materials whose surface-acoustic-wave propagation speed is fast, or methods that narrow electrode pitch have been considered. Among these, electrode pitch, being dependent on semiconductor device manufacturing technology, has limitations
Meanwhile, as a material whose surface-acoustic-wave propagation speed is fast, diamond is the fastest. In this respect, a laminated structure as is in Japanese Pub. Pat. App. No. H07-321596, in which a LiNbO3 layer is arranged on top of diamond, has been proposed. According to this literature, c-axis oriented LiNbO3 is formed onto a diamond (111) surface by an RF-magnetron sputtering technique; but with the a value expressing the c-axis orientation being below 5xc2x0, (under 4xc2x0, furthermore), the c-axis orientation cannot be said to be all that satisfactory. Likewise, Japanese Pub. Pat. App. No. H05-78200 discloses a method of synthesizing a Li(NbxTa1xe2x88x92x)O3 (0xe2x89xa6xxe2x89xa61) thin film onto a sapphire substrate. Although the obtaining of a single-phase Li(NbxTa1xe2x88x92x)O3 (0xe2x89xa6xxe2x89xa61) thin film is disclosed in this literature, nothing is disclosed as to the quality of the thin film produced. Furthermore, in Japanese Pub. Pat. App. No. H05-319993 a surface-acoustic wave device in which a LiNbO3 layer is formed onto sapphire is proposed. Nonetheless, although the LiNbO3 layer""s orientation is satisfactory, with propagation speed being some 4500 m/s because the acoustic velocity of sapphire compared with diamond is slower by some 30%, performance is unsatisfactory compared with devices employing diamond. Given this situation, being able to form a piezoelectrically satisfactory Li(NbxTa1xe2x88x92x)O3 (wherein 0xe2x89xa6x less than 1) thin film onto diamond should lead to improved device performance.
In that regard, LiNbO3 single crystals or LiTaO3 single crystals are grown industrially by the Czochralski method (CZ method). This method is carried out by selecting a congruent-melting (congruent) composition such that as the crystal proceeds to harden from the melt, the composition does not change. In LiNbO3 as a congruent composition, the atomic proportion of Li to Nb in the compositionxe2x80x94the ratio of their atomic percentagesxe2x80x94is 0.942, which is off-balanced toward Nb from the Li/Nb=1.0 that is its stoichiometric composition. The percentages of Li and Nb affect the characteristics of LiNbO3 monocrystals. In a situation in which, for example, they are off-balanced toward Nb as in the congruent composition just noted, if strong light such as a laser beam is shone on LiNbO3, a so-called optical damage phenomenon in which the refractive index alters will be apparent A need therefore arises to sacrifice the optical characteristics of the LiNbO3 itself and enhance the optical-damage resistance by the addition of MgO or the like.
With Li(NbxTa1xe2x88x92x)O3 (0xe2x89xa6xxe2x89xa61) thin films as well, the percentages of Li and NbxTa1xe2x88x92x are crucial, and in order to bring out favorable piezoelectric, electrochemical, and nonlinear-optical characteristics, the atomic constituent proportion of Li to NbxTa1xe2x88x92x (also referred to simply as xe2x80x9cLi/NbxTa1xe2x88x92x ratioxe2x80x9d hereinafter) must be controlled to be a value of over 0.80 and under 1.10. Wherein LiNbxTa1xe2x88x92xO3 thin films are fabricated by laser ablation as aforementioned, a tendency for Li to decrease from the Li and NbxTa1xe2x88x92x percentages of the raw-material targets is apparent.
Therefore, in order to suppress eduction of LiNbO8, etc. and obtain single-phase LiNbO3 when forming LiNbO3 thin films by laser ablation, making the Li constituent of the raw-material targets excessive is known. Japanese Pub. Pat. App. No. H05-78200, for instance, devises making the target Li to Nb proportion over 1.5 and under 3.5 when forming a LiNbO3 thin film onto a sapphire substrate. It has been found, however, that if by this method LiNbO3 is formed onto a diamond substrate, the Li/Nb proportion in the thin film at substrate temperatures of under 1000xc2x0 C. turns out to be over 1.10. In then measuring by X-ray diffraction, Li3NbO4 has appeared; and even with the single-phase LiNbO3, the optical characteristics under optical-damage phenomena have differed from those in which Li/Nb is from 0.80 to 1.10.
Likewise, with Li(NbxTa1xe2x88x92x)O3, wherein it has plural polarization regions, because domains whose piezoelectric constants differ cancel each other out and the overall piezoelectric characteristics do not hold, carrying out a polarization process, either when forming a Li(NbxTa1xe2x88x92x)O3 thin film over diamond or after it is formed, is necessary. Nonetheless, post-crystallization polarizing of Li(NbxTa1xe2x88x92x)O3 is known to be problematic. In general, the polarization of a ferroelectric substance can be aligned if after heating it beyond the Curie temperature, an electric field greater than the coercive electric field is applied to the crystal. In the case of LiNbO3, however, in addition to its coercive electric field being 21 kV/mm, the 1210xc2x0 C. that is the Curie temperature is close to the 1250xc2x0 C. that is the melting point of LiNbO3. Therefore, heating LiNbO3 so that it does not melt to above the Curie temperature and applying to the crystal an electric field greater than the coercive electric field is difficult.
Likewise, because at room temperature the polarization-reversing electric field strength is close to the avalanche-current-generating electric field strength, there is a danger that avalanche current will be generated during polarization, destroying the crystal. A method of carrying out polarization in bulk LiNbO3 crystals without insulator breakdown occurring despite avalanche current being generated to a certain extent, by applying a pulsed electric field to electrodes that have been furnished on either end face of the crystal, has been devised. Nevertheless, in a composite substrate of an LiNbO3 thin film and polycrystalline diamond, which is an insulating substance, when avalanche current is generated, electrical load concentrates at the interface of the LiNbO3 thin film and the diamond, and polarization cannot be carried out efficiently and moreover insulator breakdown ends up occurring. Given these circumstances, in composite substrates in which a LiNbO3 thin film is formed onto a diamond substrate, carrying out polarization simultaneously during deposition to build the LiNbO3 thin film is desirable.
Furthermore, although throughout the past betterments have been rendered in the c-axis orientation of formative Li(NbxTa1xe2x88x92x)O3 thin films, even with better c-axis orientation, the piezoelectric characteristics of the formed thin film are not always satisfactory. Li(NbxTa1xe2x88x92x)O3 thin films have polarization along the c-axis direction. In some c-axis oriented thin films, the +/xe2x88x92 of the polar directions is mixed depending on the domain. In such cases, piezoelectric characteristics are manifested locally; but if the thin film is examined as a whole, the piezoelectric characteristics turn out to be nil, and the characteristics if it is made into a circuit element turn out to be unobtainable.
The present invention was devised in order to overcome the foregoing problematic points. In particular, it presents a diamond substrate on which a c-axis oriented, moreover piezoelectrically favorable Li(NbxTa1xe2x88x92x)O3 (wherein 0xe2x89xa6xxe2x89xa61) thin film is formed, and a manufacturing method therefor.
The present invention is the finding that a c-axis oriented, piezoelectrically favorable Li(NbxTa1xe2x88x92x)O3 thin film built onto a diamond substrate can be obtained by forming a thin film of Li(NbxTa1xe2x88x92x)O3 (wherein 0xe2x89xa6xxe2x89xa61)xe2x80x94being a thin film that is a piezoelectricxe2x80x94onto a (110)-oriented polycrystalline diamond substrate, by means of a laser ablation technique that optically gathers pulsed laser light onto a raw material target to vaporize it instantaneously. The polycrystalline diamond on which the Li(NbxTa1xe2x88x92x)O3 thin film is formed must be (110)-oriented, and must be superficially mirror-surface processed.
The xe2x80x9c(110)-oriented polycrystalline diamondxe2x80x9d just noted is the X-ray diffraction intensity I(220) of a (220) face of the polycrystal diamond being 15% or more of the sum I(220)+I(111)+I(311)+I(331)+I(400) of I(220) and the X-ray diffraction intensities I(111), I(311), I(331) and I(400) of the diamond""s other faces, which are (111), (311), (331), (400). In other words, letting the proportion of I(220) to the total of all the peak intensities be xcex3(110), then xcex3(110)=I(220)/[I(220)+I(111)+I(311)+I(331)+I(400)], and xcex3(110) is 15% or morexe2x80x94more preferably, 40% or more. (If there is no orientation, then xcex3(110) is less than 15%.)
Further, it is preferable that an epi-surface of the polycrystalline diamond be covered with an amorphous layer of 1 nm or more, 50 nm or less. A Li(NbxTa1xe2x88x92x)O3 thin film formed by laser ablation onto a polycrystalline diamond of this sort is satisfactorily c-axis oriented and has favorable piezoelectric characteristics.
Herein, c-axis orientation is defined as follows. Letting the diffraction intensities for the crystal faces of Li(NbxTa1xe2x88x92x)O3 apparent in X-ray diffraction on a Li(NbxTa1xe2x88x92x)O3 thin film be I(012), I(104), I(110), I(006) and I(400), the c-axis orientation xcex3c is expressed as xcex3c=I(006)/[I(012)+I(104)+I(110)+I(006)]. The xcex3c is desirably 50% or more, and the value thereof large enough, in order that superior piezoelectric characteristics be manifested in the Li(NbxTa1xe2x88x92x)O3 thin film.
The laser used in the laser ablation process is not particularly limited, but utilizing a laser 360 nm or less in wavelength and 1 xcexcsec or less in pulse duration is preferable. While ArF and F2 excimer lasers would be lasers of this sort, a 248 nm-wavelength KrF excimer laser in particular is preferable. The preferable laser-pulse energy density is 1 J/cm2, the preferable laser frequency some 1 to 50 Hz. Further, the polycrystalline diamond substrate temperature during laser ablation should be 400xc2x0 C. or more, but because polycrystalline diamond degenerates at over 1000xc2x0 C., below 1000xc2x0 C. is preferable. Likewise, an oxidizing atmosphere consisting of oxygen, ozone, N2O, NO2, etc. is desirable for the ambient, while a 0.1 to 100 Pa range is preferable for the atmospheric pressure. In addition, a 10 to 1000-mm range is suitable for the distance between the piezoelectric thin-film raw-material target and the polycrystalline diamond substrate.
Also, in order to have the atomic constituent proportion of Li to NbxTa1xe2x88x92x in the formed Li(NbxTa1xe2x88x92x)O3 thin film be in a range of from 0.80 to 1.10, it was found that the temperature of the polycrystalline diamond substrate, and the atomic constituent proportion Li/NbxTa1xe2x88x92x of the Li and NbxTa1xe2x88x92x in the piezoelectric thin-film raw-material target, must necessarily be within the range, plotted in FIG. 6 with these as orthogonal coordinate axes, encompassed by points A (0.9 constituent proportion, 400xc2x0 C. substrate temperature; the same indications hereinafter), B(0.9, 1000xc2x0 C.), C(2.5, 1000xc2x0 C.), D (2.5, 700xc2x0 C.), and E (1.5, 400xc2x0 C.).
Moreover, in order to obtain a Li(NbxTa1xe2x88x92x)O3 thin film of satisfactory crystallinity and orientation, the temperature of the diamond substrate and the atomic constituent proportion Li/NbxTa1xe2x88x92x in the target are controlled to be within the range in FIG. 6 encompassed by points F(1.0 constituent proportion, 400xc2x0 C. substrate temperature), G (1.0, 1000xc2x0 C.), H (1.5, 1000xc2x0 C.), and I (1.4, 400xc2x0 C.).
A xe2x80x9cLi(NbxTa1xe2x88x92x)O3 thin film of satisfactory crystallinity and orientationxe2x80x9d means a film containing no phases, such as amorphous, that are not crystallized, and wherein the (001) surface of the Li(NbxTa1xe2x88x92x)O3 thin film is parallel to the substrate (so-called c-axis orientation). In the X-ray diffraction pattern for a c-axis oriented Li(NbxTa1xe2x88x92x)O3 thin film, 2xcex8 for its (006) peaks is 39xc2x0. The c-axis orientation being high means that the value of the aforementioned xcex3c is 50% or more.
The present invention is further characterized in that in the laser ablation technique, a one electrode is installed in the environs of the polycrystalline diamond substrate, and with the diamond substrate as the other electrode, the film is formed while a bias voltage is applied across the one electrode and the substrate. Doing so allows a Li(NbxTa1xe2x88x92x)O3 (wherein 0xe2x89xa6xxe2x89xa61) thin film to be deposited while it is polarized, and enables a Li(NbxTa1xe2x88x92x)O3 (wherein 0xe2x89xa6xxe2x89xa61) thin film whose piezoelectric characteristics are favorable to be formed onto a diamond substrate.
It is desirable that the one electrode as noted above be disposed in parallel with the film-deposition surface of the diamond substrate. Thus disposing it puts the c-axis of the Li(NbxTa1xe2x88x92x)O3 thin film in parallel with the orientation of the electric field, which facilitates uniform polarization within the diamond substrate surface. In order to enlarge the electric field, the distance between the one electrode and the diamond substrate is desirably 20 mm or less.
In laser ablation, moreover, routinely the deposited film surface on the substrate is disposed normal to the flight direction of the ejected particles. In the present invention, however, the diamond substrate may be situated roughly parallel to the flight path. The foregoing one electrode in this case also is disposed so as be roughly parallel with the diamond substrate. By situating in this way the diamond substrate and the one electrode in parallel with the particles"" flight direction, the particles will not pass through the electrode, thereby making it possible to suppress the in-mixing of impurities that originates in the electrode. Because the particles"" flight path is roughly in a perpendicular orientation to the target surface, the diamond substrate and the one electrode are desirably disposed within an angular range of xc2x110xc2x0 with respect to the perpendicular direction through the target surface. Beyond this range, the diamond substrate becomes shadowed as it were, and the film-formation speed drops drastically, or the effectiveness with which the particles are not let pass through the one electrode is lessened.
The foregoing one electrode is desirably a metal filament material strung into a mesh or else a grate. Inasmuch as particles ejected by the laser ablation can pass through the electrode if it is thus configured, the electrode can be disposed in an arbitrary location situating the electrode in between the target within the film-deposition device and the diamond substrate. The foregoing electrode may also be a metal sheet in a situation in which it and the diamond substrate are disposed roughly in parallel with the particles"" flight direction.
Laser ablation has the following advantages compared to sputtering and other film-deposition techniques. Forming a Li(NbxTa1xe2x88x92x)O3 (wherein 0xe2x89xa6xxe2x89xa61) thin film by exposing to oxygen plasma the diamond of the substrate is possible without damaging its surface; in-mixing of impurities is scant; because power concentration is high, Li(NbxTa1xe2x88x92x)O3 activated by excitation may be employed in film formation; and film-deposition speed may be freely adjusted. These advantages of laser ablation, the diamond""s orientation and surface condition in the diamond substrate, as well as the effects of the electric field should enable film-deposition of a Li(NbxTa1xe2x88x92x)O3 (wherein 0xe2x89xa6xxe2x89xa61) thin film whose c-axis orientation and piezoelectric characteristics are favorable.